Calculate string length

helloworld.asm

@@ -4,17 +4,33 @@
 ; Run with: ./helloworld
 
 SECTION .data
-msg     db      'Hello World!', 0Ah     ; assign msg variable with your message string
+msg     db      'Hello, brave new world!', 0Ah     ; assign msg variable with your message string
 
 SECTION .text
 global  _start
 
 _start:
-    mov     edx, 13     ; number of bytes to write - one for each letter plus 0Ah (line feed character)
+    mov     ebx, msg        ; move the address of our message string into EBX
-    mov     ecx, msg    ; move the memory address of our message string into ecx
+    mov     eax, ebx        ; move the address in EBX into EAX as well (Both now point to the same segment in memory)
-    mov     ebx, 1      ; write to the STDOUT file
+
-    mov     eax, 4      ; invoke SYS_WRITE (kernel opcode 4)
+nextchar:
+    cmp     byte [eax], 0   ; compare the byte pointed to by EAX at this address against zero (Zero is an end of string delimiter)
+    jz      finished        ; jump (if the zero flagged has been set) to the point in the code labeled 'finished'
+    inc     eax             ; increment the address in EAX by one byte (if the zero flagged has NOT been set)
+    jmp     nextchar        ; jump to the point in the code labeled 'nextchar'
+
+finished:
+    sub     eax, ebx        ; subtract the address in EBX from the address in EAX
+                            ; remember both registers started pointing to the same address (see line 15)
+                            ; but EAX has been incremented one byte for each character in the message string
+                            ; when you subtract one memory address from another of the same type
+                            ; the result is number of segments between them - in this case the number of bytes
+
+    mov     edx, eax        ; EAX now equals the number of bytes in our string
+    mov     ecx, msg        ; move the memory address of our message string into ecx
+    mov     ebx, 1          ; write to the STDOUT file
+    mov     eax, 4          ; invoke SYS_WRITE (kernel opcode 4)
     int     80h
-    mov     ebx, 0      ; return 0
+    mov     ebx, 0          ; return 0
-    mov     eax, 1      ; SYS_EXIT
+    mov     eax, 1          ; SYS_EXIT
     int     80h